8061  'tZWiM 

'A  'N  ' 


THE  ELECTRO-DEPOSITION  OF  BRASS 
FROM  CYANIDE  SOLUTIONS 

BY 

EARL  GROVER  STURDEVANT 


THE  ELECTRO-DEPOSITION  OF  BRASS 
FROM  CYANIDE  SOLUTIONS 

By 

EARL  GROVER  STURDEVANT 


A  THESIS 

Submitted  to  the  Faculty  of  the  Graduate 
School  of  the  University  of  Michigan 
in    partial    fulfillment    of    the    re- 
quirements  for  the  degree  of 
Doctor  of   Philosophy. 


1920 


ACKNOWLEDGMENT 

The  writer  wishes  to  express  his  sincere  appre- 
ciation for  the  kind  advice  and  instruction  during 
ttee  progress  of  this  work,  and  for  the  criticism 
C6ftfhe  thesis  rendered  by  Dr.  Alfred  L.  Ferguson, 
/3.trwfnose  suggestion  and  under  whose  supervision 
this  investigation  was  carried  out. 

To  Professor  S.  Lawrence  Bigelow,  for  his 
most  valuable  criticism  of  this  thesis,  sincere 
thanks  are  also  due. 


TABLE  OF  CONTENTS. 

I.     Introduction    5 

II.     Theoretical 6 

III.  Materials  and  Method  of  Manipulation 10 

IV.  Metal  Ratio 11 

V.     Metal   Content    16 

VI.     Current  Density 17 

VII.     Temperature    19 

VIII.     Sodium  Cyanide 20. 

Acid  Substances 23 

Alkaline  Substances 27 

Neutral  Substances   31 

XII.     Brass  as  Anodes 33 

XIII.  Single  Potential  of  Brass   35 

XIV.  Summary   38 


458724 


THE  ELECTRO-DEPOSITION  OF  BRASS  FROM  CYANIDE 
SOLUTIONS. 


Introduction. 

A  survey  of  the  literature  on  the  electro-deposition  of  brass 
shows  that  brass  has  been  obtained  from  cyanide  solutions  which 
vary  decidedly  in  ratio  of  copper  to  zinc,  metal  content,  cyanide 
content,  alkalinity,  current  density,  and  temperature.  This  fact 
is  clearly  evident  from  the  table  compiled  by  C.  W.  Bennett 
(Trans.  Amer.  Electrochem.  Soc.  (1913),  23,  251),  which  is  a 
summary  of  brass  plating  solutions  that  have  been  recommended. 
Calculations  from  the  figures  given  in  this  table  show  that  the 
composition  of  these  solutions,  expressed  in  grams  per  liter,  varies 
from  076  to  47.77  g.  of  copper;  1.59  to  23.38  g.  of  zinc;  0.00  to 
165.00  g.  of  potassium  cyanide;  0.00  to  180.00  g.  of  alkaline  sub- 
stances ;  and  0.00  to  90.00  g.  of  slightly  acid  substances.  The 
extreme  variation  in  metal  ratio  is  shown  by  solutions  1  and  12 
in  this  table.  In  solution  1  the  ratio  is  10  parts  of  copper  to  80 
parts  of  zinc;  in  solution  12  the  ratio  is  10  parts  of  copper  to  1 
part  of  zinc. 

The  electro-deposition  of  brass  has  been  investigated  by  F. 
Spitzer  (Zeitsch.  Electrochem.  (1905),  11,  172),  S.  Field  (Trans. 
Faraday  Soc.  (1909),  9,  172),  A.  W.  Davison  (Jour.  Phys.  Chem. 
(1914),  18,  488),  and  A.  Honig  (Zeitsch.  Electrochem.  (1916), 
22,  286).  They  worked  under  decidedly  different  experimental 
conditions  as  well  as  with  solutions  of  widely  different  composi- 
tion. These  different  experimental  conditions  may,  to  some  ex- 
tent at  least,  account  for  the  poor  agreement  of  their  results.  The 
lack  of  conformity  is  readily  apparent  from  Table  I. 

TABLE  I. 


Grams  Copper 
per  Liter 

Grams  Zinc 
per  Liter 

Ratio  of  Copper 
to  Zinc 

Percent  Copper 
Deposited 

Investigator 

6.35 

6.53 

0.97    H-^ 

73.0 

Spitzer 

317 

9.85 

0.32 

70.0 

'« 

34.25 

22.00 

1.56 

96.2 

Field 

34.25 

31.00 

1.10 

93.5 

" 

17.12 

15.50 

1.10 

86.6 

" 

6.80 

22.00 

0.37     11 

47.8 

" 

50.80 

7.10 

7.15 

71.5 

Davison 

6  EAR!,  GROVER  STURDEVANT. 

This  table  shows  the  influence  of  metal  content  and  metal  ratio 
on  the  composition  of  the  deposit.  It  is  seen  that  the  copper 
varies  from  3.17  to  50.80  g.  per  liter;  the  zinc  from  6.53  to  31.00 
g.  per  liter;  and  the  total  metal  content  from  12.88  to  65.25  g. 
per  liter.  The  slight  influence  which  the  metal  content  has  on 
the  composition  of  the  deposit  is  shown  by  the  fact  that  Spitzer 
obtained  a  deposit  of  70  percent  copper  from  a  solution  which 
contained  3.17  g.  of  copper,  while  Davison  obtained  a  deposit 
of  only  71.5  percent  copper  from  a  solution  which  contained  50.80 
g.  of  copper.  Field  found  that  by  diluting  a  solution  which  gave 
a  deposit  of  93.5  percent  copper  with  an  equal  volume  of  water, 
the  percentage  of  copper  in  the  deposit  was  reduced  to  86.6. 
•From  this  it  is  evident  that  factors  other  than  the  absolute  metal 
content  and  the  ratio  of  copper  to  zinc  in  the  solution  influence 
the  composition  of  the  deposit  to  a  marked  degree. 

A  similar  analysis  of  the  results  obtained  by  them  on  the  in- 
fluence of  cyanide  content,  alkalinity,  current  density,  and  tem- 
perature makes  clearly  evident  the  fact  that  our  present  knowledge 
of  the  brass  plating  solution  is  decidedly  indefinite  and  incomplete. 
The  lack  of  specific  data  regarding  the  influence  of  each  of  these 
factors,  together  with  the  need  for  such  data,  owing  to  the  exten- 
sive use  of  the  brass  plating  solution  commercially,  makes  it 
appear  that  a  thorough  investigation  of  the  influence  of  each  of 
these  factors  is  highly  desirable. 

Each  factor  in  some  way  affects  the  metal  ion  concentration  of 
the  solution.  All  the  effects  produced  by  changes  in  metal  ratio, 
metal  content,  cyanide  content,  alkalinity,  current  density,  and 
temperature  must  ultimately  result  from  changes  in  metal  ion 
concentration.  Since  the  influence  of  all  of  these  factors  is  the 
result  of  a  change  in  metal  ion  concentration,  it  seems  that  a 
study  of  metal  ion  concentrations  is  the  most  logical  method  of 
attacking  this  problem.  Such  a  study  is  most  successfully  made 
by  potential  measurements.  For  this  reason,  therefore,  the  in- 
fluence of  each  of  these  factors  upon  the  single  potentials  of 
copper  and  zinc  in  their  cyanide  solutions  was  determined. 

Theoretical. 

A  difference  of  potential  exists  between  an  ionizable  substance 
and  a  solution  of  its  ions.  The  magnitude  of  this  potential  in  the 
case  of  a  metal  is  given  by  the  well-known  equation 


THE   ELECTRO-DEPOSITION   OF   BRASS. 


NQ  p 

In  this  equation  £  is  the  potential  in  volts  which  a  metal  of  solu- 
tion tension  P  and  valence  N  shows  against  a  solution  of  one  of 
its  salts  in  which  the  osmotic  pressure  of  the  metal  ions  is  repre- 
sented by  p.  The  absolute  temperature  is  represented  by  T, 
Faraday's  constant  by  Q,  and  the  gas  constant  by  R.  E  is  the 
equilibrium  potential  which  must  be  exceeded  before  metal  ions 
will  deposit  from  such  a  solution.  Metals  which  become  posi- 
tively charged  in  a  solution,  normal  with  respect  to  their  ions, 
will  be  known  as  electro-positive  in  this  discussion.  Those  which 
assume  a  negative  charge  will  be  known  as  electro-negative. 

In  the  electro-deposition  of  a  metal  from  an  aqueous  solution 
of  its  ions  it  is  necessary  to  keep  in  mind  that  hydrogen  ions  are 
also  present.  If  the  applied  potential  exceeds  a  certain  value 
hydrogen  will  be  deposited.  The  magnitude  of  this  potential  is 
given  by  the  equation 

Pf 


i  < 

where  eh  is  the  hydrogen  over-voltage  for  the  particular  electrode, 
P'  is  the  solution  potential  of  hydrogen  and  p'  is  the  osmotic  pres- 
sure of  hydrogen  ions  in  the  solution.  The  other  letters  in  the 
equation  have  the  same  significance  as  in  equation  (1).  Accord- 
ing to  equation  (  1  )  the  potential  necessary  to  separate  the  metal  is 

RT  P 

E  =  -  log,        -  . 

NQ  p 

It  is  evident  that  if  £3  is  greater  than  £  metal  only  will  be  de- 
posited, if  £i  is  equal  to  £  hydrogen  and  the  metal  will  separate 
simultaneously,  and  if  £x  is  less  than  £  hydrogen  alone  will  be 
deposited.  This  reasoning  applies  equally  well  for  any  other 
positive  ion  in  the  solution. 

From  this  it  follows  that,  from  a  solution  in  which  there  are 
a  number  of  metallic  ions,.  only  that  metal  is  deposited  which  is 
the  most  electro-positive.  For  example,  in  an  ordinary  solution 
containing  copper  and  zinc  ions,  copper  alone  is  deposited,  since 
copper  is  nearly  a  volt  more  positive  than  zinc. 


8  EARL  GROVER  STURDEVANT. 

In  order  to  separate  two  metals  simultaneously,  as  pointed  out 
above,  the  single  potentials  which  these  metals  show  in  the  solu- 
tion must  be  nearly  equal.  Since  the  solution  potential  of  a  metal 
has  a  definite  characteristic  value,  it  follows  from  equation  (1) 
that  the  potential  which  a  metal  shows  in  a  particular  solution 
must  depend  upon  the  osmotic  pressure  of  its  ions  in  that  solu- 
tion. If  in  the  above  solution  of  copper  and  zinc  sulphate,  the 
concentration  of  the  copper  ions  be  made  continuously  smaller, 
the  copper  becomes  less  electro-positive  and  thus  approaches  the 
value  of  zinc  in  the  same  solution.  If  this  process  could  be  con- 
tinued long  enough  zinc  and  copper  would  deposit  simultaneously. 
The  practical  impossibility  of  accomplishing  this  by  dilution  is 
indicated  by  a  calculation  from  equation  (1)  which  shows  that 
the  copper  ion  concentration  in  such  a  solution  is  only  about 
1  x  10-38  N. 

The  decrease  in  metal  ion  concentration,  by  dilution,  decreases 
also  the  conductivity.  Both  the  low  metal  content  and  the  low 
conductivity  thus  produced  are  undesirable.  When  metal  content 
is  low  there  is  no  ready  source  for  the  renewal  of  the  ions  re- 
moved from  about  the  cathode,  and  they  are  practically  all 
removed  by  the  passage  of  only  a  small  quantity  of  electricity. 
When  the  conductivity  is  low  a  greater  amount  of  energy  is  con- 
sumed in  the  deposition  of  a  given  amount  of  metal.  In  order, 
therefore,  to  decrease  the  ion  concentration  of  a  solution  without 
decidedly  decreasing  the  conductivity,  and  at  the  same  time  to  pro- 
vide a  ready  source  of  ion  supply,  substances  are  used  which  form 
complex  ions  containing  the  metal.  For  example,  the  addition  of 
sodium  cyanide  to  a  solution  of  copper  sulphate  forms  the  com- 
plex salt  Na2Cu(CN)3,  which  dissociates  until  the  following 
equilibrium  is  established  — 

Na2Cu(CN)i  ^2Na+  +   'Cu(CN)^2Nar  -f   Cu+   +  3CN. 

Equilibrium  is  established  with  an  extremely  small  concentration 
of  copper  ions.  Zinc  forms  a  similar  complex  salt  Na2Zn(CN)4 
and  a  similar  equilibrium  is  established  — 

**  2Nar  +  ZnCN          2Na+   +  Zn++  +  4CN. 


In  case  copper  or  zinc  are  removed  from  such  solutions,  new 
ions,   formed  by   dissociation,   immediately   replace   them.     The 


THE   ELECTRO-DEPOSITION   OF   BRASS.  9 

copper  complex  compound  is  much  more  stable  than  the  zinc, 
hence,  by  the  addition  of  sodium  cyanide  to  a  solution  of  copper 
and  zinc  sulphates,  the  copper  ion  concentration  is  decreased  de- 
cidedly more  than  the  zinc  (Spitzer,  loc.  cit.).  This  means  that 
the  single  potentials  of  zinc  and  copper  approach  each  other  on 
the  addition  of  sodium  cyanide.  By  the  addition  of  large  quan- 
tities of  sodium  cyanide  the  potentials  become  equal,  or  copper 
may  even  become  more  electro-negative  than  zinc.  (Spitzer,  loc. 
cit.  S.  P.  Thompson,  Chem.  News  (1887),  55,  300.)  The  cop- 
per ion  concentration  in  such  a  solution  is  about  10~38  N. 

From  this  it  appears  possible  to  prepare  a  solution  of  the  com- 
plex cyanide  compounds  of  copper  and  zinc  in  which  these  metals 
show  the  same  single  potentials.  During  the  process  of  elec- 
trolysis of  such  a  solution  the  single  potentials  of  copper  and  zinc 
are  different  from  the  equilibrium  value.  This  results  from  the 
change  in  ion  concentration  immediately  surrounding  the  elec- 
trodes. The  actual  ion  concentration  in  the  vicinity  of  the  elec- 
trodes depends  upon  the  current  density,  temperature,  rate  of 
stirring,  and  rate  of  ionization  of  the  complex  compounds.  Other 
conditions  remaining  the  same,  it  is  evident  that  an  increase  in 
current  density  results  in  an  increase  in  the  metal  ion  concen- 
tration at  the  anode  and  a  decrease  at  the  cathode.  Increase  in 
temperature,  by  increasing  diffusion,  counteracts  to  some  extent 
these  changes  in  ion  concentration.  Stirring  and  ionization  also 
tend  to  maintain  a  uniform  ion  concentration. 

The  value  of  electrolytically  deposited  brass  is  determined 
chiefly  by  its  color  and  this  depends  largely  upon  its  composition. 
A  brass  of  65  percent  copper  and  35  percent  zinc  has,  under 
favorable  conditions,  a  satisfactory,  bright  yellow  appearance. 
In  order  that  the  composition  of  the  solution  which  deposits  brass 
of  this  composition  may  remain  unaltered  the  metals  must  dis- 
solve from  the  anode  in  this  ratio  and  the  efficiency  of  solution 
must  be  the  same  as  that  of  deposition.  These  are  the  conditions 
which  must  be  fulfilled  by  an  efficient  brass  plating  solution  and 
the  maintenance  of  these  conditions  constitutes  the  chief  problem 
of  brass  plating.  The  ratio  in  which  copper  and  zinc  dissolve  or 
deposit  is  determined  by  the  ratio  of  their  ion  concentrations  in 
the  solution  immediately  surrounding  the  electrode.  The  prob- 
lem, then,  consists  in  establishing  this  ratio  and  in  maintaining 


10  KARL  GROVKR  STURDEVANT. 

it  by  the  proper  regulation  of  factors  which  influence  ion  con- 
centration. 

Materials  and  Method  of  Manipulation. 

The  cells  used  were  600  c.c.  beakers.  There  were  two  anodes 
and  one  cathode  in  each  cell.  One  anode  was  a  sheet  of  cast  zinc 
50  by  50  by  2  mm.  and  the  other  was  a  sheet  of  electrolytic  cop- 
per of  the  same  dimensions.  These  were  supported  parallel  to 
each  other  and  about  8  cm.  apart.  The  cathode  was  a  platinum 
sheet  50  mm.  square,  supported  midway  between  the  anodes. 
Between  each  anode  and  the  positive  terminal  of  the  battery 
were  placed  a  coulometer,  a  variable  resistance,  and  an  ammeter. 
A  return  wire  from  the  cathode  to  the  negative  terminal  of  the 
battery  completed  the  circuit. 

The  solution  was  vigorously  stirred  by  a  glass  rod  provided 
with  a  paddle  about  3  cm.  long.  The  rod  was  rotated  about 
1200  r.  p.  m. 

The  potential  measurements  were  made  by  the  compensation 
method  with  a  small  Leeds  and  Northrup  potentiometer  graduated 
to  0.5  millivolt.  A  suitable  galvanometer  was  used  as  zero  in- 
strument. As  a  standard  comparison  potential,  a  cadmium  cell 
with  a  potential  of  1.01845  at  20°  C.  was  used.  Normal  calomel 
electrodes  were  used  as  reference  electrodes.  To  avoid  including 
in  the  potential  measurements  some  of  the  potential  fall  due  to 
the  current  in  the  solution,  the  ends  of  the  calomel  electrodes  were 
placed  as  near  the  anodes  as  possible  and  on  the  side  opposite 
to  the  cathode.  The  end  of  the  calomel  electrode  against  which 
the  cathode  potential  was  measured  was  placed  near  the  edge  of 
the  cathode  for  the  same  reason. 

Only  the  highest  grade  C.  P.  materials  were  used  in  this  work. 
Analysis  showed  that  they  did  not  require  further  purification. 

Two  cells  were  set  up  as  above  described,  and  all  determina- 
tions were  run  in  duplicate.  Only  one  set  of  results  for  each 
determination,  however,  is  recorded  in  the  tables.  At  the  begin- 
ning of  each  determination  the  anodes  and  cathodes  were  weighed 
and  placed  in  the  cells.  The  calomel  electrodes  were  placed  in 
position  and  a  set  of  potential  readings  taken  before  the  current 
was  turned  on.  The  current  density  was  0.15  ampere  per  sq.  dm. 
at  each  anode  and  0.30  ampere  at  the  cathode.  The  current  was 


THE  ELECTRO-DEPOSITION  OF  BRASS.  II 

allowed  to  flow  for  two  hours,  giving  a  cathode  deposit  of  about 
0.6  g.  With  the  current  flowing,  three  sets  of  potential  readings 
were  taken,  one  immediately  after  the  current  was  turned  on, 
one  at  the  middle  of  the  determination,  and  one  just  before  the 
current  was  turned  off.  Immediately  after  the  current  was  turned 
off  another  set  of  potential  readings  was  taken.  All  these  read- 
ings are  recorded  in  the  tables. 

At  the  end  of  the  determination  the  anodes  and  cathode  were 
immediately  removed  from  the  solution,  washed  with  tap  water, 
with  distilled  water,  and  finally  with  alcohol.  They  were  then 
allowed  to  dry  at  room  temperature  before  weighing.  From  the 
loss  in  weight  of  the  anodes  and  the  quantity  of  electricity  as 
shown  by  the  coulometers,  the  efficiencies  of  anode  corrosion  were 
calculated. 

The  deposit  was  dissolved  in  dilute  nitric  acid  and  the  copper 
determined  electrolytically.  Zinc  in  the  deposit  was  obtained  by 
difference.  From  these  data  the  percentage  of  copper  in  the  de- 
posit and  the  cathode  efficiency  were  calculated.  The  efficiency 
of  corrosion  or  deposition  is  obtained  by  dividing  the  weight  of 
material  dissolved  or  deposited  by  the  weight  of  material  equiva- 
lent to  the  amount  of  electricity  shown  by  the  coulometers. 

Unless  otherwise  stated,  the  temperature  was  25°  C.  ±  1°. 
A  closer  regulation  of  the  temperature  was  not  considered  neces- 
sary. 

Metal  Ratio. 

It  has  already  been  pointed  out  that  there  are  many  factors 
which  influence  the  metal  ion  concentration  of  the  brass  plating 
solution  and  the  nature  of  the  deposit.  These  factors  were  indi- 
vidually studied  and  the  exact  influence  of  each  determined. 
Metal  ratio  was  the  first  of  these  factors  to  receive  special  atten- 
tion. 

It  seemed  logical  to  begin  with  a  solution  containing  the  two 
metals  in  about  the  same  ratio.  The  solution  first  used  contained 
4.5  grams  of  copper  and  6.7  grams  of  zinc  per  liter  in  the  form 
of  complex  cyanides.  (This  is  the  metal  content  of  solution  4 
in  Bennett's  (loc.  cit.)  table.)  No  other  constituents  were  pres- 
ent. The  solution  was  electrolyzed  for  twenty  hours  and  the 
changes  which  resulted  in  it  observed.  These  observations  are- 
recorded  in  Table  II. 


12 


EARL  GROVER  STURDEVANT. 


The  many  disadvantages  of  this  solution  are  readily  apparent. 
The  results  show  at  first  a  high  anode  solution  due  to  the  cor- 
rosive action  of  the  excess  cyanide.  The  excess  cyanide  also  pro- 
duces a  high  cathode  polarization  which  causes  the  deposition  of 
large  quantities  of  hydrogen  and  consequently  a  low  cathode 
efficiency.  As  electrolysis  is  continued  the  excess  cyanide  is  re- 
moved ;  this,  as  is  to  be  expected,  increases  the  cathode  efficiency 
and  decreases  the  zinc  anode  efficiency.  The  potentials  of  both 
copper  and  zinc  decrease.  In  cyanide  solutions  the  solution  pres- 
sure of  these  metals  is  greater  than  the  osmotic  pressure  of  their 
ions,  hence  a  decrease  in  the  potentials  indicates  an  increase  in 

TABU;  II. 


Efficiencies 

Potential  Measurements 

Per- 

Grams 

Row 

Anodes    ' 

cent  of    of  Free 
Copper    Cyan- 
r-jtVi     i  i*1  De-     ide  ner 

Total 
Hrs. 
Run 

With 
Current 

Without 
Current 

Character  of 
Deposit 

posit 

Liter 

Copper     Zinc 

Copper 

Zinc 

Copper 

Zinc 

1 

108.0 

87.2 

6.9 

45.5 

7.4 

2 

0.951 

1.231 

1.073 

1.272  Dull  Red 

2 

107.2 

100.4 

75.8 

58.9 

5.4 

4 

0.906 

1.115 

1.059 

1.248 

«        « 

3 

107.2     129.3 

91.2 

57.4 

4.5 

6 

0.822 

1.100 

0.987    1.241 

Uniform  Yellow 

4 

109.3      96.7 

96.9 

39.9 

3.6 

8 

0.834 

1.129 

0.992  1  1.228 

«           « 

5 

114.4 

94.1 

100.2 

36.5 

3.3 

10 

0.814 

1.149 

0.983    1.231 

Purple 

6 

107.5 

116.2 

98.7 

43.7 

3.5 

12 

0.679 

-0.531 

0.826    0.891 

Dark  Brown 

7 

107.6 

57.1 

95.3 

48.3 

3.5 

14 

0.660 

-0.595 

0864;  0.940  i       " 

8 

106.5 

49.2 

96.4 

44.7 

3.0 

16 

0.655 

-0.575 

0.870 

0.795 

«           « 

9 

108.8 

55.8 

97.0 

45.8 

2.9 

18 

0.654 

-0.573 

0.817 

0.705 

10 

108.8 

60.3 

97.1 

47.7 

2.6 

20 

0.606 

-0.676 

0.804 

0.665 

«           « 

metal  ion  concentration.  The  change  in  ion  concentration  is  more 
pronounced  in  the  case  of  zinc  than  of  copper ;  so  much  more,  in 
fact,  that  the  copper  becomes  more  electro-negative  than  zinc. 
The  change  appears  to  be  most  pronounced  at  a  definite  stage  in 
the  electrolysis.  At  this  stage  a  white  precipitate  of  zinc  cyanide 
appears  on  the  zinc  anode  and  the  efficiency  of  this  anode  sud- 
denly decreases.  This  change  in  relative  ion  concentration  is 
probably  responsible  for  the  fluctuation  in  the  percentage  of 
copper  in  the  deposit. 

The  percentage  of  copper  in  all  the  deposits  obtained  here  was 
lower  than  that  required  for  a  good  brass.  This  low  percentage 
of  copper  indicated  that  the  copper  content  of  the  solution  was  too 


THIv   ELECTRO-DEPOSITION   OF   BRASS. 


low.  It  seemed,  therefore,  that  by  adding  copper,  a  solution 
might  be  obtained  which  would  give  a  deposit  of  suitable  copper 
content  in  the  presence  of  sufficient  free  cyanide  to  produce 
efficient  anode  corrosion.  The  addition  of  copper,  as  the  complex 
cyanide,  to  the  electrolyzed  solution,  showed  that  the  percentage 
of  copper  in  the  deposit  could  be  increased  in  this  way.  The 
results  of  such  additions  are  given  in  Table  III. 

A  solution  of  the  composition  of  that  used  in  Table  II  was  elec- 
trolyzed nearly  to  the  stage  at  which  the  decided  change  in  ion 
concentration  occurred.  It  then  gave  the  values  in  row  1  of  Table 
III.  To  this  solution  copper  sulphate,  with  the  equivalent  sodium 
cyanide,  was  added  in  sufficient  quantity  to  double  the  original 


III. 


Row 

Efficiencies 

Per- 
cent of 
Copper 
in  De- 
posit 

Grams 
of  Cop- 
per per 
Liter 
of  Sol. 

Potential  Measurements 

Character  of 
Deposit 

Anodes 

Cath. 

With 
Current 

Without 
Current 

Copper 

Zinc 

Copper     Zinc 

Copper 

Zinc 

1 

2 
3 

4 

5 

103.8 
104.7 
108.1 
109.1 
109.7 

95.5 
106.5 
102.8 
100.3 
104.7 

96.7 
80.1 
76.5 
99.0 
98.7 

36.7 
59.1 
62.4 
66.1 
80.7 

446 
8.92 
13.37 
17.83 
23.40 

0.640     1.015 
0.750    0.967 
0.861    0.918 
0.869    0.994 

0.854  !  0.991 

i 

0.865 
0.964 
0.926 
0934 
0.902 

1.189 
1.130 
1.125 
1.009 
1.027 

Dull  yellow 

«      "         « 

Bright    " 
Dull        " 
Bright    ' 

copper  content.  The  effect  this  had  on  the  solution  is  indicated 
in  row  2.  Further  additions  of  copper  were  made  and  the  results 
obtained  are  given  in  rows  three,  four  and  five.  The  increase  in 
copper  concentration  is' accompanied  by  the  following  changes: 
(1)  the  percentage  of  copper  in  the  deposit  increases,  and  as 
might  be  expected  the  nature  of  the  deposit  becomes  more  satis- 
factory;  (2)  the  potential  of  zinc  decreases,  indicating  an  increase 
in  zinc  ion  concentration;  (3)  the  potential  of  copper  increases, 
indicating  a  decrease  in  the  copper  ion  concentration.  This  de- 
crease in  copper  ion  concentration  is  not  exceptional,  since  in  all 
electrolytes  the  ion  concentration  becomes  less  with  increase  in 
concentration  of  the  electrolyte  beyond  a  certain  point.  This 
phenomenon  in  the  case  of  copper  has  also  been  observed  by 
Spitzer  (loc.  cit.).  Copper  tends  to  form  complex  compounds 


GROVER  STURDEVANT. 


containing  more  cyanide  than  Na2Cu(CN)3 ;  and  since  the  copper 
complex  is  more  stable  than  the  zinc,  the  latter  will  tend  to  be 
decomposed  in  a  solution  which  contains  both.  This  will  allow 
more  zinc  cyanide  to  dissociate  and  hence  increase  the  zinc  ion 
concentration. 

From  the  table  it  appears  that,  with  a  solution  which  contained 
23.4  g.  of  copper  and  6.7  g.  of  zinc,  a  deposit  of  the  desired 
copper  content  can  be  obtained  with  approximately  100  percent 
anode  and  cathode  efficiencies.  Accordingly,  a  solution  which 
contained  24.0  g.  of  copper  and  6.7  g.  of  zinc  was  prepared,  but 
it  was  found  that  this  solution,  when  boiled  for  a  half  hour,  gave 
a  deposit  of  only  60,0  percent  copper.  All  solutions  prepared  at 

TABUS  IV. 


Efficiencies 

Potential  Measurements 

Per- 

Grams 

Row           Anodes 

cent  of 
Copper 
in  De- 

of Cop- 
per p€r 
Liter 

With  Current 

Without  Current 

Character 
of  Deposit 

A         A 

A         A 

Copper 

\_ain. 
Zinc 

; 

posit 

of  Sol. 

Cath. 

Cath. 

Copper  1    Zinc 

Copper 

Zinc 

1      105.5 

117.4      97.6 

60.1 

24.46 

0.900 

0.878 

1.444 

0.974 

1.076 

0.973 

DullYellow 

2 

107.2 

104.4      96.4 

67.7 

28.46 

0.874 

0.942 

1.463 

0.963 

1.085    0.987 

«        « 

3 

104.0 

90.8    100.0 

75.2 

32.46 

0.892 

0.922 

1.388 

0.940 

1.030 

0.960 

"     Red 

4 

102.8 

96.3  I    98.8 

80.9 

37.00 

0.875 

0.942 

1.319 

0.936 

1.037 

0.987 

«        « 

room  temperature  had  a  characteristic  red-brown  color  which 
was  removed  by  boiling  and  also  by  continued  electrolysis.  From 
these  results  it  appeared  that  there  was  a  close  connection  between 
the  removal  of  the  red-brown  color  and  the  decrease  in  the  per- 
centage of  copper  in  the  deposit,  and  that  the  changes  within  the 
solution,  which  are  responsible  for  the  decrease  in  the  percentage 
of  copper  in  the  deposit,  may  be  brought  about  by  boiling  the 
solution  as  well  as  by  continued  electrolysis.  The  potentials  ob- 
tained in  the  boiled  solution  corresponded  to  those  obtained  in  an 
unboiled  solution  of  lower  copper  content.  This  is  in  agreement 
with  the  results  of  Honig  (loc.  cit.)  who  found  that  the  separa- 
tion potential  of  copper  from  its  cyanide  solution  was  increased 
to  a  marked  extent  by  boiling  the  solution.  The  exact  nature  of 
the  change  produced  is  unknown. 


THE  ELECTRO-DEPOSITION   OF  BRASS.  15 

These  results  indicated  that  in  order  to  obtain  the  desired  per- 
centage of  copper  in  the  deposit  still  greater  concentrations  of 
copper  were  necessary.  Four  solutions,  containing  respectively 
24.5,  28.5,  32.5  and  37.0  g.  of  copper,  were  prepared  and  boiled 

TABLE  V. 


Efficiencies 

Nun>                           Potential  Measurements 

Row 

Per- 
cent of 
Copper 

her  of 
Hours 

Character  of 

Anodes 

With  Current 

Without  Current 

r*   *u 

in  De- 

Elec- 

A          A 

ueposu 

Copper 

Zinc 

Lath. 

posit 

tro- 

lyzed 

Cath. 

Anodes 

Cath. 

Copper     Zinc 

Copper 

Zinc 

1 

107.0 

85.2 

93.6 

64.0 

2 

0.891    0.976 

1.420 

0.955 

1.064 

1.000  Dull  light 

0.897    0.960 

0.979 

1.080 

yellow 

0.903  i  0.980 

2 

105.6 

87.9 

98.4 

64.9 

6 

0.883    0.950 

1.381 

0.956 

1.039 

0.996  Dull  yellow 

0.883    0.950 

0.967 

1.051 

0.884    0.948 

3 

105.6 

103.3 

98.5 

65.9 

10 

0.858    0.980 

1.370 

0.941 

1.045 

0.987       " 

0.864    0.983 

0.958 

1.032 

0.867    0.986 

4 

103.9 

92.2 

99.0 

68.4 

18 

0.873    0.956 

1.365 

0.947 

1.030 

0.981       " 

0.869    0.939 

0.955 

1.054 

0.867    0948 

5 

104.1 

93.4 

98.3 

68.2 

34 

0.875    0.954 

1.346 

0.951 

1.075    0.983      " 

0.875    0.983 

0.957 

1.108 

0.879    0.978 

6 

104.4 

96.2 

98.1 

63.3 

54 

0.887    1.035 

1.368 

0.963 

1.085    0.997      " 

0.890    0.985 

0.973 

1.119 

0.887    0.959 

7 

105.0 

107.9 

103:3 

50.8 

72 

0.853    0900 

1.371 

0.935 

1.090 

1.023  Bright  yellow 

0.841     0.869 

0.950 

1.107 

0.837    0.891 

8 

104.1 

106.0  ;  104.2 

50.3 

74 

0.847    0.805  '  1.358 

0.933 

1.100 

0.994 

«          « 

0.836    0.807 

0.931 

1.107 

0.820    0.803 

9 

103.2 

108.8  j  103.5 

42.1 

88 

0.755    0.377    1.394 

0.860 

1.201 

1.195       "      silver 

0  763    0.384 

0.891 

1.215 

color 

0770    0.389 

1 

previous  to  electrolysis.  All  these  solutions  contained  6.7  g.  of 
zinc  per  liter.  These  solutions  on  electrolysis  gave  the  results 
recorded  in  Table  IV.  The  data  here  given  show  that  an  increase 
in  the  ratio  of  copper  to  zinc  in  the  solution  increases  the  per- 
centage of  copper  in  the  deposit  A  solution  containing  28.5  g. 
of  copper  per  liter  is  necessary  under  these  conditions  to  give  a 
deposit  containing  65  percent  of  copper. 


1 6  EARL  GROVER  STURDEVANT. 

To  determine  whether  or  not  a  solution  of  this  composition 
was  suitable  for  use  in  the  remainder  of  this  investigation,  two 
samples  were  electrolyzed  for  a  period  of  eighty-eight  hours.  At 
various  stages  in  the  electrolysis  determinations  were  made  of  the 
cathode  and  anode  efficiencies  and  of  the  percentage  of  copper 
in  the  deposit.  The  usual  potential  readings  were  taken.  The 
results  are  recorded  in  Table  V. 

These  results  show  that  a  solution  of  this  composition  gives 
satisfactory  anode  and  cathode  efficiencies  for  a  period  of  about 
fifty  hours.  The  high  copper  anode  efficiencies  may  be  accounted 
for  by  the  solvent  action  of  the  cyanide  on  copper.  The  zinc 
anode  efficiencies  recorded  are  not  as  reliable  as  those  for  copper, 
because  in  nearly  all  cases  copper  deposited  on  the  zinc  anode 
and  decreased  the  accuracy  of  the  results.  The  constancy  of  the 
ion  concentrations  during  this  period  is  indicated  by  the  constancy 
of  the  potential  measurements. 

Satisfactory  bright  yellow  brass  deposits  are  obtained  during 
the  first  fifty  hours.  Later  the  deposits  become  slightly  dull  and 
at  the  end  of  eighty-eight  hours  they  have  a  pink  appearance.  It 
is  noticed  that  even  in  this  solution  the  percentage  of  copper  in 
the  deposit  finally  decreases.  Since,  however,  this  solution  can 
be  relied  upon  to  give  constant  results  for  a  period  of  about  fifty 
hours,  it  is  satisfactory  for  use  in  the  remainder  of  this  work. 

Metal  Content. 

The  influence  of  a  decrease  in  metal  content  of  the  electrolyte 
was  next  determined.  Two  dilutions  were  made ;  one  by  adding 
an  equal  volume  of  water  to  the  original  solution,  and  the  other 
by  adding  an  equal  volume  of  water  to  the  diluted  solution.  The 
usual  potential  measurements  and  efficiency  determinations  were 
made  and  the  data  recorded  in  Table  VI. 

The  potential  measurements  show  that  as  the  solution  is  diluted 
there  is  practically  no  change  in  zinc  ion  concentration,  but  an 
increase  in  copper  ion  concentration.  The  increase  in  the  degree 
of  ionization  of  the  copper  salt  on  dilution  must  increase  the  cop- 
per ion  concentration  more  than  dilution  decreases  it.  The  in- 
crease in  copper  ion  concentration  would  normally  increase  the 
percentage  of  copper  in  the  deposit,  but  the  increase  in  cathode 
polarization  shows  that  the  metal  ions  are  removed  by  electro- 


THE  ELECTRO-DEPOSITION   OF   BRASS.  17 

deposition  more  rapidly  than  they  are  furnished  by  dissociation 
in  these  dilute  solutions.  This  results  in  the  deposition  of  rela- 
tively greater  amounts  of  the  more  electro-negative  metal,  zinc. 
In  the  second  dilution,  however,  this  effect  is  overcome  by  the 
more  rapid  increase  in  copper  ion  concentration.  The  increase  in 
cathode  polarization  is  accompanied  by  the  usual  decrease  in 
cathode  efficiency. 

During  the  electrolysis  of  the  dilute  solutions  white  deposits 
of  metal  cyanides  form  on  the  anodes.     This  indicates  that  the 


TABLE  VI. 


Efficiencies 

Potential  Measurements 

Per- 
cent of 

centra-               With  Current                     Without  Current             Character 

Row 

Anodes 
Cath. 

Copper 
in  De- 
posit 

tion, 
Origi- 
nal 

nf  Dpnrxsit 

Anodes 

Anodes 

Cath. 

Cath. 

Copper     Zinc 

Copper 

Zinc 

Copper 

Zinc 

1 

1032 

103  9      97  1 

666 

0.872 

1.007 

1.485    0946 

1046 

1  022           null    rpH 

0.867 

1.000 

1.495    0.976 

1.052 

0.869 

1.006 

1.500 

2 

108.0 

108.3 

92.2 

59.5 

y* 

0.844 

0.983 

1.555    0.937     1.135    0.996           "  erav 

0.831 

0.963 

1.564    0.950     1.080 

0.830 

0.930 

1.550 

3 

68.2 

73.8 

81.3      61.0 

% 

0.795 

0.915 

1.619    0.900 

1.100 

0.980           «    « 

-0.535 

—0.760  '  1.625    0.921 

1.140 

—0.550 

—0.690  !  1.621 

cyanide  concentration  is  too  low,  and  accounts  for  the  low  anode 
efficiencies.  The  results  obtained  here  are  due  not  only  to  the 
decrease  in  metal  content  but  also  to  the  decrease  in  the  cyanide 
concentration.  It  is  clearly  evident  from  these  results  that  the 
original  solution  is  the  most  satisfactory. 

Current  Density. 

The  original  solution,  when  electrolyzed  at  different  current 
densities,  gave  the  results  recorded  in  Table  VII. 

The  data  show  that  both  the  cathode  efficiency  (Curve  1,  Plate 
I)  and  the  percentage  of  copper  in  the  deposit  (Curve  3,  Plate  I) 
uniformly  decrease  and  that  the  cathode  polarization  (Curve  2, 


i8 


EARI,  GROVER  STURDEVANT. 


Plate  I)  uniformly  increases  with  increase  in  current  density. 
The  potential  measurements  show  that  the  ion  concentrations  re- 
main practically  unchanged.  The  decrease  in  cathode  efficiency 
and  percentage  of  copper  in  the  deposit  are  to  be  expected,  since 

E7ff.f 


0.7     fa 


m     <2b     m     m    A\n?. 


urve  5 
W 


Eff- 
^urve  1 

m- 


PLATE  I.     Influence  of  Current  Density. 

1.  Change  in  cathode  efficiency  with  increase  in  current  density. 

2.  Potential   of  copper  anode  with   current  flowing. 

3.  Potential  of  cathode  with  current  flowing. 

an  increased  cathode  polarization  produces  the  deposition  of  rela- 
tively larger  quantities  of  zinc  and  hydrogen.  Neither  a  maxi- 
mum percentage  of  copper  nor  a  minimum  cathode  efficiency, 
such  as  was  obtained  by  Spitzer  (loc.  cit.),  is  here  observed.  This 
is  probably  due  to  the  difference  between  the  metal  content  of 
this  solution  and  that  used  by  Spitzer. 


THE:  ELECTRO-DEPOSITION  OF  BRASS. 


The  character  of  the  deposit  becomes  less  satisfactory  with  in- 
crease in  current  density  beyond  0.3  ampere  per  sq.  dm.  of  cathode 
surface.  For  this  reason  a  current  density  of  0.3  ampere  is  used 
in  the  remainder  of  this  work. 

Temperature. 

The  work  of  other  investigators  shows  that  an  increase  in  tem- 
perature increases  the  percentage  of  copper  in  the  deposit  and  the 

TABLE  VII. 


Efficiencies 
Per- 

Cur- 

Potential  Measurements 

Row 

cent  of 
Conner 

Den-                With  Current                    Without  Current        ,    Character  of 

Cath.      inn^' 

sity 
Amp./           Anodes 

Anodes 

Cop 

per     Zinc 

dm.2    r 
Copper 

Zinc 

Cath. 
Copper 

Zinc 

Cath.  ; 

1      104 

L6      91.4     100.5      78.6 

0.2      0.907 

0.950 

1.328    0.978 

1.070 

0.991   Dull  red 

0893 

0.943 

0.966 

1.042 

0.894 

0.937 

2      102 

!.9      89.2      98.2      64.8 

0.3 

0.852 

0.941 

1.440    0.966 

1.091 

0.989  Dull  red,  not 

0.853 

0.937 

0.959 

1.064 

uniform 

0.849 

0.941 

3      10= 

5.8      97.7 

97.0      53.8 

0.5 

0.862 

0.968 

1.500    0.983 

1.130 

1.009   Dull   gray 

0.858 

0.949 

0.985 

1.082 

! 

0.859 

0.938 

4     102.0     1122 

95.7      52.9 

0.7 

0.858 

0942 

1.560    0.984 

1.100    1.004      " 

0.858 

0.954 

0.985 

1.056 

0.864 

0.961 

i 

5      100.9     101.6 

92.4      52.1 

1.0 

0.795 

0.540 

1.651    0.986 

1.097    0.999       " 

0.791 

—0.744 

0.979 

1.093 

I 

0.787 

—0.880 

cathode  efficiency.  Since  the  compositions  of  the  solutions  used 
by  them  were  much  different  from  the  one  here  used,  and  since 
no  potential  measurements  were  made,  it  was  considered  desirable 
to  obtain  a  series  of  measurements  at  different  temperatures.  The 
results  are  given  in  Table  VIII. 

It  is  observed  that  with  increase  in  temperature  there  is  a  slight 
gradual  increase  in  the  potentials  of  copper  and  zinc  in  the  solu- 
tion (Curves  1  and  2,  Plate  II).  This  is  probably  accounted  for 
by  the  more  rapid  diffusion  of  the  dissolved  material  away  from 


20 


GROVER  STURDEVANT. 


the  electrodes  at  higher  temperatures.  The  pronounced  decrease 
in  the  cathode  polarization  with  increase  in  temperature  (Curve 
3,  Plate  II)  probably  results  from  the  rapid  diffusion  of  metal 
ions  to  the  cathode.  Accompanying  the  decrease  in  cathode 
polarization  is  the  usual  increase  in  the  percentage  of  copper  in 
the  deposit  (Curve  4,  Plate  II). 

These  results  are  in  good  agreement  with  those  obtained  by 
Field  (loc.  cit.)  and  others. 


E  M.'F 


urve 


tt- 


0 


-3$ 


-80 


PLATE  II.     Influence  of  change  in  Temperature. 

1.  Potential   of  copper  without  current. 

2.  Potential  of  zinc  without  current. 

3.  Potential  of  cathode  with  current  flowing. 

4.  Change  in  the  percentage  of  copper  in   the   deposit. 


Sodium  Cyanide. 

The  results  obtained  when  sodium  cyanide  was  added  to  the 
solution  are  given  in  Table  IX. 

A  study  of  the  literature  gives  the  impression  that  an  increase 
in  free  cyanide  increases  the  anode  efficiency.  These  results, 
however,  fail  to  show  this  effect.  In  fact,  the  efficiency  of  the 
zinc  anode  tends  to  decrease  with  increase  in  free  cyanide.  As 
the  concentration  of  the  free  cyanide  increases,  the  potentials  of 
the  anodes  and  the  cathode  increase  (Curves  1,  2  and  3,  Plate 
III)  ;  this  indicates  a  decrease  in  metal  ion  concentration.  This 


THE  ELECTRO-DEPOSITION   OF   BRASS. 


21 


decrease  in  metal  ion  concentration  makes  necessary  an  increase 
in  cathode  potential  in  order  to  deposit  the  metals.  The  increase 
in  cathode  potential  causes  the  deposition  of  relatively  larger  quan- 
tities of  hydrogen  and  results  in  a  decrease  in  cathode  efficiency. 
The  percentage  of  copper  in  the  deposit  shows  a  peculiar 
change.  There  is  first  a  decided  decrease,  then  a  gradual  increase  as 
the  concentration  of  free  cyanide  increases  (Curve  4,  Plate  III). 

TABLE  VIII. 


Efficiencies 

Potential  Measurements 

Row 

Per- 
cent of 
Copper 
in  De- 
nnsit 

Tem- 
pera- 
ture 
Cent. 

Character  of 
Deposit 

Anodes 

Cath. 

With  Current                    Without  Current 

Anodes                                Anodes 

Copper 

Zinc 

"     Cath.                                '     Cath 
Copper     Zinc                      Copper     Zinc 

1 

103.1 

88.0 

98.1      64.2 

26° 

0.865    0.943  '  1.456    0.960     1.054    0.992    Dull  red 

0.868    0.960                0.957     1.043 

0.868    0.933 

2 

105.4 

93.2 

100.2 

80.9 

40° 

0.927 

0.983     1.371     0.976     1.083     1.023    Bright  red- 

0.928 

0.960                0.982     1.053 

yellow 

0.936    0.962 

3 

104.8 

72.0 

100.7      83.9 

50° 

0.958    0.991     1.327     1.000 

1.070     1.037    Bright  red- 

0.965 

0.997                1.007 

1.056 

yellow 

0.972 

1.007 

4 

104.6 

83.7 

99.2 

85.2 

60° 

1.008 

.984    1.253     1.026 

1.056     1.040    Bright  red- 

1.006 

1.007               1.031 

1.056 

yellow 

1.009 

1.009 

5 

103.8 

57.7 

99.6      90.2 

70° 

1.041 

1.045     1-200     1.046 

1  078     1.062    Bright  red- 

1.039 

1.044               1.055 

1.078 

yellow 

1.038 

1.040 

This  could  not  be  accounted  for  by  observed  changes  in  potential 
and  was  at  first  thought  to  be  due  to  error  in  analysis.  Two  other 
sets  of  determinations,  the  data  for  which  are  not  given,  showed 
a  similar  behavior.  It  thus  appears  that  there  are  two  concentra- 
tions of  free  cyanide  at  which  a  brass  containing  65  percent  of 
copper  (a  satisfactory  composition)  can  be  obtained  from  this 
solution.  Since  the  lower  cyanide  concentration  gives  a  much 
better  cathode  efficiency  and  an  equally  good  anode  efficiency,  it 
is  the  one  used  in  the  remainder  of  this  work. 


22  KARL  GROVER  STURDEVANT. 


CM 


"N 


G  ^ 

+  £  + 

tc  '^  O 


•5 

a 


t     a     NK 

d         "^  i        !|^ 

>  .^4- 


THE  ELECTRO-DEPOSITION   OE   BRASS.  23 

Acid  Substances. 

The  greater  part  of  the  copper  and  zinc  in  their  cyanide  solu- 
tions is  present  as  Na2Cu(CN)3  and  Na2Zn(CN)4  (F.  Kunchert, 
Zeitsch.  anorg.  Chem.  (1904),  41,  337).  A  study  of  the  various 
possible  equilibria  that  may  exist  in  such  solutions  shows  that 
acid  and  alkaline  substances  may  influence  the  extent  to  which 


E.M.F 
i  /  /) 

// 

s*3 

<±LlL 

l.UU 
1   rr/j 

/ 

K 

/ 

Cum  i 

1  ,JU 
i  A/} 

/ 

/  (/ 
1      /- 

i  •  rv 

1    ~Zf] 

^ 

• 

UJ 

I.JV 
1    -Jf] 

^-Z 

_      t  ; 

o(J 

1  t/iU 
1  If] 

> 

^^ 

> 

X.' 

I.IU 
1  f)  ft 

X"' 

^ 
^' 

1  *l/U 
f]  Q/] 

/ 

!5 

U.JU 

5          1 

0           1 

s      a 

0        2 

5     Gr. 

NoCN/L. 

PI.ATE  III.      Influence  of   Sodium   Cyanide. 

1.  Potential  of  copper  anode  without  current. 

2.  Potential  of  zinc  anode  without  current. 

3.  Potential  of  cathode  with   current. 

4.  Percentage  of  copper  in  the  deposit. 


the  indicated  reactions  take  place  and  hence  the  metal  ion  con- 
centration in  these  solutions.      (See  preceding  page.) 

It  is  apparent  that  the  addition  of  alkaline  substances  forces 
reactions  (I  a  2)  and  (II  a  2)  to  the  left;  this  increases  the  con- 
centration of  molecular  sodium  cyanide.  Reactions  (I  a  1)  and 
(II  a  1)  are  then  forced  to  the  right,  increasing  the  concentration 


24  EARL  GROVER  STURDEVANT. 

of  sodium  and  cyanide  ions.  This  increase  in  sodium  and  cyanide 
ions  from  the  sodium  cyanide  causes  a  decrease  in  the  concen- 
tration of  copper  and  zinc  ions. 

The  addition  of  acid  substances  forces  reactions  (I  a  2)  and 
(II  a  2)  to  the  right  with  the  formation  of  slightly  dissociated 
hydrocyanic  acid,  thus  increasing  the  cyanide  ion  concentration 


IX. 


Efficiencies 

Potential  Measurements 

Per- 

Grams 

Row 

Anodes 
Cath. 

cent  of  of  Free 
Copper    Cyan- 
in  De-  ide  per 
posit      Liter 

With  Current 

Without  Current 

Charac- 
ter of 
Deposit 

Anodes 

Anodes 

Copper 

Zinc    j 

Copper     Zinc 

Lath. 

Copper     Zinc 

Cath. 

1         103.2 

103.9      97.1 

66.6 

7.0    0.850    0.956 

1.439 

0.921     1.052 

0.989 

Dull  red 

0.870    0.965 

1.435 

0.955    1.040 

0.880    0.981 

1.436 

2      103.6 

87.5      94.7 

60.4 

12.0 

0.879    1.024 

1.539 

0.990    1.125 

1.035 

"  gray 

0.882    1.009 

1.542 

1.002    1.055 

0.883    1.006 

1.546 

3      103.7 

74.1 

79.6 

62.3 

17.0 

0.943    1.060 

1.595 

1.057    1.169 

1.095 

0.926    1.067 

1.590 

1.062     1.142 

0.929    1.060 

1.595 

4 

102.7      86.3      51.3 

64.8     22.0    0.971     1.111 

1.650    1.126    1.189 

1.146      "     " 

0.960    1.105 

1.642    1.111     1.183 

0.950    1.104 

1.644 

5 

103.3      86.5 

38.2 

65.5 

27.0    1.065    1.195 

1  696    1.200    1.245 

1.174      "     " 

1.051     1.166 

1.707    1.164    1.220 

1.013    1.130 

1.712 

produced  from-  the  sodium  cyanide.  This  permits  a  greater  dis- 
sociation of  the  copper  and  zinc  complexes  and  results  in  an  in- 
crease in  the  concentration  of  copper  and  zinc  ions.  It  should  be 
mentioned  here  that,  since  the  stability  of  the  copper  complex  is 
greater  than  that  of  the  zinc  (Kunchert,  loc.  cit.)  the  effects  in 
the  two  cases  are  not  of  the  same  magnitude. 

As  a  result  of  these  considerations  all  the  substances  other  than 
sodium  cyanide  which  have  been  added  to  brass  plating  solutions 
are  arbitrarily  classified  as  acid,  neutral,  or  alkaline.  It  is  believed 
that  their  influence  can  be  explained  by  their  action  as  acid,  neu- 


THE  ELECTRO-DEPOSITION  OF  BRASS.  25 

tral,  or  alkaline  substances  upon  the  above  equilibria  existing  in 
brass  plating  solutions. 

A  set  of  experiments  was  carried  out  with  each  of  the  three 
acid  substances,  ammonium  chloride,  sodium  hydrogen  sulphite, 
and  boric  acid.  Ammonium  chloride  and  sodium  hydrogen  sul- 


TABLE  X. 
Sodium  Hydrogen  Sulphite. 


Efficiencies 

Potential  Measurements 

Per- 
cent of 
Copper 
in  De- 
posit 

Grams 
NaHSO3 
per 
Liter 

Character 
of  Deposit 

Row          Anodes 

Cath. 

With  Current                    Without  Current 

Anodes 

Anodes 
r*  it 

/"*    *l» 

Copper 

Zinc 

Copper 

Zinc 

Latn. 
Copper 

Zinc 

Lath. 

1      106.9 

106.8 

95.2 

62.6 

0.0 

0.864 

0.970 

1.390    0.952 

1.038 

0.984 

Dull  Red 

0.876 

0.967 

0.964 

1.049 

0.880 

0.951 

2      101.6 

111.7 

97.8 

72.3 

5.0      0.865 

0.907 

1.358    0.942 

.985 

0.963 

«      « 

0.868 

0.917 

0.945 

1.004 

0.873 

0.912 

3      103.7 

124.2 

99.1 

77.0 

10.0 

0.858 

0.883 

1.307    0.914 

0.981 

0.940 

«<      « 

0.866 

0.890 

0.926 

0.996 

0.867 

0.880 

4      105.9 

148.3 

99.0 

80.6 

15.0 

0.841 

0.860 

1.220    0.887 

0.924 

0.920 

«      « 

0.844 

0.863 

0.901 

0.901 

0.849 

0.853 

5      109.4 

128.3 

99.9 

85.7 

20.0 

0.828 

0.821 

1.144    0.878 

0.940 

0.899 

«      « 

0.828 

0.485 

0.883 

0.910 

0.830 

0.153 

6      111.5 

112.0 

97.1 

96.9        30.0       0.797 

—0.723 

1.082    0.837 

0.899 

0.875        "      " 

0.770 

—0.723 

0.843 

0.901 

0.470 

—0.723 

phite  were  selected  because  they  are  the  most  commonly  used  acid 
substances.  Boric  acid  was  used  because  its  action  can  be  only 
that  of  a  weak  acid.  The  results  obtained  by  the  addition  of 
variable  amounts  of  each  of  these  substances  are  recorded  in 
Tables  X,  XI  and  XII. 

The  general  influence  of  the  three  substances  is  the  same. 
Sodium  hydrogen  sulphite  produces  the  greatest  effect  and  ammo- 
nium chloride  the  least.  Since  sodium  hydrogen  sulphite  is  only 


26 


EARL  GROVER  STURDEVANT. 


slightly  acid,  the  effect  observed  here  seems  greater  than  was  to 
be  expected.  This  point  is  given  further  consideration  following 
the  discussion  of  alkaline  substances.  The  potentials  in  all  cases 
show  a  regular  decrease  (Curves  1  and  2,  Plates  IV  and  V), 
which  indicates  an  increase  in  metal  ion  concentration.  This  in- 

TABLE  XL 
Ammonium  Chloride. 


Efficiencies 

Potential  Measurements 

Per-      r 

Row 

Anodes 

Cath. 

cent  of    Ggtns 
Copper         * 
"poSft-      filer 

With  Current 

Anodes 
rvtii 

Without  Current 

Anodes 
n~4.i. 

Character 
of  Deposit 

Copper 

Zinc 

Copper 

Zinc 

Copper 

Zinc 

1 

103.1 

87.6 

97.3 

65.6        0.0 

0.892 

0.968 

1.445 

0.962    1.062    0994 

Dull  red 

0.895 

0.965 

0.973     1.039 

0.891 

0.966 

i 

2 

103.1 

58.8 

98.2 

66.3 

3.0 

0.880 

0.950 

1.421 

0.967    1.052    0993 

«      « 

0.882 

0.950 

0.970    1.038 

0.887 

0.940 

3 

103.1 

82.9 

97.5 

68.9 

5.0 

0.898 

0.962 

1.436 

0.959 

1.046    0.992 

«      „ 

0.904 

0.958 

0.971 

1.034 

0.903 

0.930 

4 

102.6 

81.9 

98.0      68.7 

7.0    0.893 

0.975 

1.423 

0.968 

1.034    0.996        "      " 

0.899 

0.969 

0.973 

1.016 

0.903 

0.935 

5 

92.2 

72.4 

97.8      71.9 

11.0 

0.909 

0.903 

1.400 

0.956 

1.011     0.993        "      " 

0.904 

0.909 

0.965 

1.021 

0.900 

0.934 

6 

100.9 

57.0 

97.6      72.5 

13.0 

0.903 

0.915 

1.395 

0.962    0.998    0.998        "      " 

0.903 

0.930 

0.970     1.021 

0.902 

0.921 

crease  in  metal  ion  concentration  is  to  be  expected  from  the  above 
conclusions  regarding  the  influence  of  acid  substances  upon  the 
equilibria  which  exist  in  the  solution.  The  decrease  in  cathode 
polarization  (Curve  3,  Plates  IV  and  V)  is  accompanied  by  the 
customary  increase  in  the  percentage  of  copper  in  the  deposit 
(Curve  4,  Plates  IV  and  V).  It  is  concluded  from  these  results 
that  no  advantage  is  to  be  gained  by  the  addition  of  any  of  these 
substances  to  a  solution  of  the  composition  here  used.  A  solution 


THE  ELECTRO-DEPOSITION  OF  BRASS.  27 

of  lower  copper  content,  which  normally  gives  a  deposit  too  low 
in  copper,  may  be  made  to  give  a  deposit  of  the  desired  copper 
content  by  the  addition  of  acid  substances. 


TABLE  XII. 
Boric  Acid. 


Efficiencies 
Per- 

Grams                            Potential  Measurements 

_r                                                                        ~  —  

cent  of 

OI 

Boric               With  Current 

Without  Current 

Character 

Row          Anodes                          in°  De* 

Acid 
per              Anodes 

Anodes 

of  Deposit 

posit 

T      .                            [ 

Cath. 

Cath. 

Copper 

Zinc 

Copper 

Zinc 

Copper 

Zinc 

1       103.4 

94.6      97.8      65.7 

00     0.831 

0.918 

1.467 

0.895    1.072 

0.997 

Dull  red 

0.854 

0.925 

0.925    1.063 

0.857 

0.975 

2     j  102.2 

111.3      98.4      67.4 

2.0     0.846 

0.940 

1.468 

0.924     1.014 

0.981 

„      « 

0.849 

0.929 

0.943    1.038 

0.850 

0.926 

3 

99.8 

115.9 

99.2 

67.7 

40    0.856 

0.920 

1.460 

0.930    1.028 

0.973 

«      a 

.  0.856 

0.916 

0.943 

1.023 

0.857 

0.920 

4 

99.2 

117.8 

99.2 

71.2 

8.0     0.838 

0.924 

1.458 

0.917 

1.017 

0.959 

«      « 

0.838 

0.916 

0.926 

1.000 

0.841 

0.910 

5 

100.6 

111.8 

99.4 

71.4 

15.0  ^  0.830 

0.884 

1.432    0.906 

0.994 

0.950 

«            « 

0.830 

0.875 

0.921 

1.000 

0.833     0.882 

6 

101.2 

115.9 

98.8 

73.7 

20.0     0.827     0.892 

1.412    0.886 

0.977     0.938 

«      « 

1 

0.835    0.885 

0.908 

0.993 

•    1 

0.841    0.881 

7 

98.3 

110.3 

100.6 

78.6 

25.0     0.845    0.829 

1.354    0.892 

0.981     0.931 

«      « 

0.845    0.832 

0.910 

0.975 

0.845    0.825 

Alkaline  Substances. 

The  alkaline  substances  first  used  were  ammonium  hydroxide 
and  sodium  carbonate.  The  results  produced  by  the  addition  of 
variable  amounts  of  each  of  these  substances  to  the  original  solu- 
tion are  recorded  in  Tables  XIII  and  XIV. 

The  increase  in  potential  of  both  copper  and  zinc  (Curves  1 
and  2,  Plates  VI  and  VII)  shows  that  the  metal  ion  concentra- 


28 


EARL  GROWER  STURDEVANT. 


tions  decrease  with  increase,  in  the  alkalinity  of  the  solution. 
This  decrease  in  metal  ion  concentration  produces  an  increase  in 
cathode  polarization  (Curve  3,  Plates  VI  and  VII)  and  thus  a 
decrease  in  the  percentage  of  copper  in  the  deposit  (Curve  4, 

TABLE  XIII. 
Ammonium  Hydroxide. 


Efficiencies 

Potential  Measurements 

i    Per-       Eauiva- 

Row 

Anodes 

cent  of 
Copper 
Cath.     in  De' 

lent  of 
NH4OH 
per 

With  Current 

Without  Current 

Character  of 
Deposit 

Anodes 

Anodes 

posit 

Copper 

Zinc 

[  Copper     Zinc 

Copper     Zinc 

1 

103.8 

95.5  !   99.3      63.9 

0.0 

0.830      .975 

1.425    0.907     1.090     1.005    Dull  red 

0.846 

.972 

1.435    0.935    1.028 

0.842 

.992 

2 

103.5 

93.4 

98.4 

58.0 

0.15 

0.918    1.019 

1.460    0.968     1.127     1.021    Bright  yellow 

0.919    1.011 

1.475    1-005     1.109 

0.916     1.007 

3 

103.3 

95.4 

99.6 

55.3 

0.30 

0.894     1.045 

1.468 

0.970 

1.097    0.986 

0.910 

1.040 

1.475 

1.008 

1.121 

0.916 

1.056 

4 

103.2 

95.8 

98.9 

54.6 

0.45 

0.950 

0.994    1.481 

1.020 

1.170    l.oil 

0.945 

1.018     1.485 

1.031 

1.135 

0.951 

1.005 

5 

101.9 

128.9 

99.4 

53.3 

0.60 

0.936 

1.020 

1.504 

1.021 

1.140    1003 

0.927 

1.001     1.500 

1.028 

1.198 

0.924 

0.965 

6 

102.2 

95.7 

99.3 

54.6 

0.75 

0.951 

1.055     1.484 

1.028 

1.160    0.991 

0.951 

1.023     1.483    1.032 

1:172 

0.949 

1.014 

7 

104.6 

101.3 

99.3 

53.0 

1.05 

0.979 

1.084    1.499    1.059 

1.219    1.056 

0.975 

1.090    1.474    1.047 

1.215 

0.948 

1.041 

Plates  VI  and  VII).  As  is  to  be  expected,  the  influence  of  these 
substances  is  opposite  to  that  of  acid  substances.  Ammonium 
hydroxide  has  a  greater  effect  than  sodium  carbonate,  but  the 
effect  in  both  cases  compared  with  that  of  acid  substances  is  small. 
These  substances  have  a  pronounced  effect  on  the  nature  of  the 
deposit.  A  bright-yellow  deposit  is  obtained  in  both  cases.  As 
the  percentage  of  copper  in  the  deposit  decreases  the  brass  changes 


THE   ELECTRO-DEPOSITION   OF   BRASS.  29 

from  a  dull  red  to  a  bright  yellow  at  about  58  percent  of  copper, 
and  remains  bright  yellow  throughout  the  determinations.  From 
these  results  it  appears  desirable  to  add  a  small  quantity  of  an 
alkaline  substance  to  the  solution  because  of  the  favorable  in- 
fluence it  has  on  the  nature  of  the  deposit. 

It  was  pointed  out  above  that  sodium  hydrogen  sulphite  pro- 
duces a  greater  effect  than  is  to  be  expected  from  its  acidity.    This 


E.M.F. 

/~ 

/^ 

/* 

'lurve  ^ 

^N^ 

1  * 

N 

^ 

^ 

/  ' 

^ 
/ 

\ 

/ 

# 

/  / 

\ 

\ 

III 

frf^ 

\ 

\ 

^ 

\ 

^ 

-) 

0.8     0. 

65     0. 

&      0. 

^-^ 

—  —  . 

^     0. 

J 

&.!?» 

)gen    Sulp 

PLATE   IV.      Influence   of   Sodium-  Hydrc 

hite. 

1.  Potential  of  copper  anode  without  current. 

2.  Potential  of  zinc  anode  without  current. 

3.  Potential  of  cathode  with  current. 

4.  Percentage  of  copper  in  the  deposit. 


suggested  that  it  might  have  some  influence  other  than  that  of  an 
acid  substance.  It  was  thought  possible  that  it  might  exert  a 
reducing  effect  on  the  solution  and  thus  influence  the  metal  ion 
concentration.  In  order  to  obtain  more  information  on  this  point 
two  series  of  experiments  were  performed ;  one,  by  the  addition 
of  variable  amounts  of  sodium  hydrogen  sulphate,  and  the  other, 
by  the  addition  of  variable  amounts  of  sodium  sulphite.  Sodium 
sulphite  was  used  to  obtain  the  influence  of  the  sulphite  ion  in 


3° 


KARL  GROVER  STURDEVANT. 


the  absence  of  an  acid  substance.  Sodium  hydrogen  sulphate  was 
used  to  obtain  the  influence  of  the  hydrogen  ion  in  the  absence 
of  the  sulphite  ion.  The  results  obtained  by  the  addition  of  these 
substances  to  the  original  solution  are  given  in  Tables  XV  and 
XVI. 

TABLE:  XIV. 
Sodium  Carbonate. 


Row 

Efficiencies 

Per- 
cent of 
Copper 
in  De- 
posit 

Equiva- 
lent of 
Na2C03 
per 
Liter 

Potential  Measurements 

Character  of 
Deposit 

Anodes 

Cath. 

With  Current 

Without  Current 

Anodes 

Cath. 

Anodes 

Cath. 

Copper     Zinc 

Copper 

Zinc 

Copper 

Zinc 

1 

103.0      89.2 

98.2    64.8 

0.0 

0.852 

0.941 

1.440 

0.966 

1.091 

0.989 

Dull  red 

0.853 

0.937 

0.959 

1.064 

0.849 

0.931 

2 

102.5      88.4 

98.2    57.0 

0.094    0.857 

0.930 

1.440 

0.953 

1.036 

0.999       "     reddish 

0.869 

0.998 

0.969 

1.079 

yellow 

0.878 

0.988 

3 

102.8 

97.6 

99.4 

57.62 

0.188    0.866 

0.993 

1.430 

0.966 

1.066 

0.998       "     yellow 

0.874 

0.957 

0.971 

1.079 

0.873 

0.961 

4 

103.2 

94.8 

99.4 

58.2 

0.282 

0.873 

0.952 

1.439 

0.968 

1.088 

0.998  !  Bright    " 

0.869 

0.958 

0.972 

1.107 

0:869 

0.962 

5 

102.0 

94.7 

99.3 

5&0 

0.376 

0.869 

0.981 

1.430 

0.984 

1.094 

1.000       " 

0.866 

0.972 

0.973 

1.133 

0.864 

0.976 

6 

101.5 

89.6 

98.8 

56.8 

0.564 

0.861 

0.995 

1.451 

0.996 

1.177 

1.007      " 

0.855 

0.979 

0.979 

1.099 

0.855 

0.987 

7 

101.6 

91.7 

98.7 

55.6 

0.752 

0.861 

1.015 

1.454 

1.001 

1.154 

1.004  ;   " 

0.856    1.015 

0.998 

1.125 

0.853    0.997 

The  changes  produced  by  sodium  sulphite  are  slight.  The  only 
observed  effect  is  that  of  an  extremely  weak  base.  This  indicates 
that  the  sulphite  ion  has  no  specific  influence.  Sodium  hydrogen 
sulphate  has  approximately  the  same  effect  as  sodium  hydrogen 
sulphite.  One  would  expect  sodium  hydrogen  sulphate  to  have 
the  greater  effect,  since  in  a  pure  solution  of  sodium  hydrogen 
sulphate  the  hydrogen  ion  concentration  is  distinctly  greater  than 


THE  ELECTRO-DEPOSITION  OF  BRASS.  31 

in. a  pure  solution  of  sodium  hydrogen  sulphite.  Even  though 
the  influence  of  sodium  hydrogen  sulphite  is  more  pronounced 
than  can  be  accounted  for  by  the  concentration  of  hydrogen  ions 
in  a  pure  solution,  nevertheless  it  appears  to  have  no  influence 
other  than  that  of  an  acid  substance.  In  the  preparation  of  the 
original  solution,  however,  a  quantity  of  sodium  hydrogen  sulphite 
equivalent  to  the  copper  is  beneficial  in  that  it  prevents  the  loss  of 


£^ 

^-  

L— 

-—  *~— 

<1<Cu 

/  •  i 

/  -z. 

x 

Curve  4 
t    i?  ') 

1,0 

/  9 

_^_—  — 

r—- 

-  I  

-—  —  •—  -1 

-"" 

^   oO 

/  ; 

70 
(  n 

/./ 
/  ^) 

^-—  ^ 

uu 

—  -<-_ 

+2 

' 

0. 

2         0. 

4       0. 

to             0. 

8         1. 

0  FQU/\ 

MBft// 

PLATE   V.     Influence  of  Boric  Acid. 
1.     Potential  of  copper  anode  without  current. 
2.      Potential  of  zinc  anode  without  current. 
3.     Potential  of  cathode  with  current. 
4.     Percentage  of  copper  in  the  deposit. 

cyanide  (W.  D.  Bancroft,  Jour.  Phys.  Chem.  (1905),  9,  277). 
It  is  concluded  from  these  results  that  the  changes  produced 
by  any  of  the  acid  substances  may  be  obtained  by  the  addition  of 
a  weak  acid,  and  that  ammonium  hydroxide  may  be  used  in  place 
of  any  of  the  alkaline  substances. 

Neutral  Substances. 

In  the  preparation  of  brass  plating  solutions  the  metal  content 
is  obtained  from  the  chlorides,  nitrates,  sulphates,  acetates,  car- 
bonates, or  oxides  of  copper  and  zinc.  When  sodium  cyanide  is 


32  EARL  GROVER  STURDEVANT. 

added  to  solutions  of  these  salts  the  sodium  salts  of  these  acids 
are  formed  in  the  solution.  This  is  equivalent,  in  the  case  of 
acetates,  carbonates,  or  oxides,  to  the  addition  of  alkaline  sub- 
stances ;  and  in  the  case  of  chlorides,  nitrates,  or  sulphates,  to  the 

TABLE  XV. 
Sodium  Sulphite. 


Row 

Efficiencies 

Per- 
cent of 
Copper 
in  De- 
posit 

Grams 
Na2SO3 
per 
Liter 

Potential  Measurements 

Anodes 

Cath. 

With  Current 

Without  Current            Character  of 

Anodes 

Cath. 

Deposit 
Anodes 

Path 

Copper 

Zinc 

Copper 

Zinc 

Copper     Zinc 

1 

103.0 

92.0     90.2 

63.7 

0.0      0.857 

0.903 

1.493 

0.916     1.021    0.995    Dull  red 

0.858 

0.958 

0.944     1.037                           yellow 

0.863 

0.946 

2 

102.5 

90.0     97.0 

60.5 

5.0 

0.849 

0.953 

1.504 

0.934    1.053    0.996    Slightly  dull 

0.851 

0.943 

o  040    1  nm                            11 

vcllo\v 

0.851 

0.932 

3 

102.3 

97.6     96.4 

62.7 

10.0 

0.842 

0.985 

1.498 

0.943    1.036    0996       "    "    " 

0.847 

0.973 

0.962    1.042 

0.844 

0.980 

4 

101.7 

95.7      97.5 

61.9 

15.0 

0.868    0.968 

1.493    0.953 

1.030    0.991       "    "    " 

. 

0.870    0.964 

0.960 

1.038 

0.869    0.976 

5 

100.9 

114.5 

95.4 

61.1 

20.0 

0.833    0.921     1.500 

0.934 

1.033    0.982       "    "    " 

0.850    0.910 

0.945 

1.043 

0.851    0.910 

6      100.3 

106.0 

96.0 

60.8 

25.0 

0.851    0.935    1.509 

0.940 

1.027    0.980       "    "    " 

0.857    0.936 

0.951 

1.049 

0.859    0.922 

7 

100.7 

92.5 

96.2 

60.9 

30.0 

0.853    0.939    1.520 

0.930 

1.049    0.979       "    "    " 

0.857    0.928 

0.935 

1.069 

0.855    0.917 

1 

! 

addition  of  neutral  substances.  It  was  considered  advisable, 
therefore,  to  determine  whether  these  neutral  salts  influenced  the 
action  of  the  solution.  To  do  this  a  series  of  experiments  was 
performed  in  which  various  quantities  of  sodium  sulphate  were 
added  to  the  original  solution.  The  results  obtained  by  such  addi- 
tions are  given  in  Table  XVII. 


THE  ELECTRO-DEPOSITION   OF   BRASS. 


33 


The  changes  produced  are  so  slight  that  they  are  within  the 
limits  of  experimental  error.  These  results  are  to  be.  expected 
since  it  appears,  from  the  above  discussion  of  the  equilibria  exist- 
ing in  cyanide  solutions,  that  only  substances  of  an-acid  or  alkaline 
nature  can  influence  the  metal  ion  concentration. 

TABLE  XVI. 
Sodium  Acid  Sulphate. 


Efficiencies                                                                    Potential  Measurements 
Per-         - 

cent  of     £ggg            Wi 
Row          Anodes                         Copper  Aa*l:)Uj 
Cath.     'JL2r:    Liter              ^no 

th  Current                  Without  Current            Character  of 

des                                Anodes                                 Deposit 

Copper     Zinc                                                       Copper 

Zinc                      Copper     Zinc 

1      102.6 

104.6     98.3      62.6        0.0      0.874 
0.869 
0.866 

0932     1.496    0.944     1.025    0.997    Dull  red- 
0.936     1.508    0.945     1.020                           yellow 
0.906     1.508 

2      103.2 
3      102.3 

118.8      98.3     63.7        2.5-    0.854 
0.853 
0.851 

107.5      98.3     69.0        5.0     0.857 
0.859 
0.856 

0.943     1.480    0.897     1.021     0.982    Dull  red 
0.915     1.485    0.926     1.035 
0.919     1.479 

0.905     1.491    0.902     1.018    0.971       "      " 
0.900     1.474    0.931     1.022 
0.900    1.471 

4      102.7 

110.9     98.2     75.1       10.0      0.866 
0.867 
0.862 

0.892     1.431    0.889    0.972    0.953       "      " 
0.898    1.438    0.916    0.989 
0.888     1.433 

5      103.1 

116.9     99.2     79.1       15.0     0.852 
0.851 
0.851 

0.875     1.390    0.882    0.951    0.930      "      " 
0.884     1.387    0.900    0.969 
0.888    1.385 

•6      101.3 

109.7     99.5     80.1      20.0     0.831 
0.817 
0.809 

0.853     1.371    0.861    0.933    0.912       "      " 
0.869     1.361    0.885    0.976 
0.876    1.374 

• 

Brass  as  Anodes. 

In  all  of  this  work  copper  and  zinc  anodes  were  used  to  make 
possible  the  study  of  various  factors  by  the  measurement  of 
single  electrode  potentials.  This  method  of  maintaining  the  metal 
content  of  the  solution,  however,  is  unsatisfactory.  The  zinc  be- 
comes covered  with  a  layer  of  copper  which  interferes  with  its 
normal  behavior.  During  electrolysis  this  deposit  drops  from  the 
electrode  and  collects  as  a  sediment.  Such  action  results  in  an 
uncontrollable  change  in  the  metal  content  of  the  solution. 


34 


GROVER  STURDEVANT. 


In  an  earlier  experiment  (see  Table  V)  a  solution  was  elec- 
trolyzed  for  a  period  of  eighty-eight  hours.  During  this  time  the 
current  through  each  anode  was  so  regulated  that  the  metals  dis- 
solved and  deposited  in  the  same  ratio.  At  the  end  of  this  time 
analysis  showed  that  the  solution  had  increased  in  metal  content, 
probably  due  to  the  solvent  action  of  the  cyanide,  but  the  zinc  had 
increased  relatively  more  than  the  copper.  This  reduced  the  ratio 


E.W.K 

7  ^ 

2 

ItJ 
*-""^ 

^—  r—  ' 

—  ^  

if 

^             ^J 

Curve  4 

X 

rv 

^ 

"^^  

~~r  —  ~ 

_< 

*~~4 

/«A 

1   1           v^ 

^"^^^  X 

^~^ 

r~~~?. 

2 

/ 

r—  - 

*~-l 

fa     o 

2         0 

t        0. 

6        0. 

8        / 

0  Ecjuiy. 

NMH/L 

PLATE  VI.     Influence  of  Ammonium  Hydroxide. 
1.     Potential  of  copper  anode  without  current. 
2.     Potential  of  zinc  anode  without  current. 
3.     Potential  of  cathode  with  current. 
4.     Percentage  of  copper  in  the  deposit. 

of  copper  to  zinc  in  the  solution  and  consequently  reduced  the 
percentage  of  copper  in  the  deposit. 

Experiments  were  consequently  performed  to  determine  the 
anodic  behavior  of  brass.  For  this  purpose  brass  anodes,  the  com- 
position of  which  varied  from  62.5  percent  to  85.0  percent  copper, 
were  cast  and  their  efficiencies  of  corrosion  determined.  The 
value  recorded  for  the  efficiency  in  each  case  is  the  average  of  ten 
determinations.  The  results  obtained  are  given  in  Table  XVIII. 


THE  ELECTRO-DEPOSITION    OF   BRASS. 


35 


The  brass  anodes  maintain  their  original  bright  yellow  color 
and  no  sediment  forms  during  the  electrolysis.  The  surface  of 
the  electrodes  remains  smooth  and  there  is  apparently  no  tendency 
for  zinc  to  dissolve  in  preference  to  copper,  as  might  be  expected 
from  the  difference  in  the  solution  pressures  of  these  metals ;  in 

TABLE  XVII. 
Sodium  Sulphate. 


Row 

Efficiencies 

Per- 
cent of 
Copper 
in  De- 
posit 

Grams 
Na2SO4 
per 

Liter 

Potential  Measurements 

Character  of 
Deposit 

Anodes 

Cath. 

With   Current 

Without  Current 

Anodes 
r>~*u 

Anodes 

Cath. 

Copperi  Zinc 

Copper 

V^dLU, 

Zinc 

Copper     Zinc 

1 

102.3 

95.5     91.5 

61.4 

0.0     0.860 

0.941     1.503 

0.906 

1.035 

0.991    Dull  red- 

0.854 

0.947     1.502 

0.932 

1.042 

yellow 

0.851 

0.950    1.501 

2 

99.4 

98.5     96.1 

61.7 

5.0     0.837 

0.944    1.500 

0.911 

1.038 

0.983    Slightly  dull 

0.840 

0.948    1.493 

0.929 

1.042 

yellow 

0.840 

0.915    1.491 

3 

1  102.7 

93.0 

97.3 

63.8      15.0     0.831 

0.935    1.478 

0.893 

1.042    0.980 

0.844 

0.933    1.497 

0.935 

1.048- 

0.845 

0.900     1.493 

4 

102.3 

107.3     97.6 

65.3      25.0     0.848 

0.931     1.481 

0.904 

1.052    0.976 

0.847 

0.950    1.479 

0.937 

1.029 

0.838 

0.940,    1.479 

5 

102.6 

103.5 

98.9 

66.2 

35.0     0.840 

0.934    1.453 

0.906 

1  048    0.979 

«            « 

0845 

0.939    1.454 

0.938 

1.047 

0.846 

0.943    1.455 

* 

6 

99.8 

118.6 

99.1 

65.3 

45.0 

0.825 

0.924    1.475 

0.908 

1.025    0.972 

0.826 

0.925     1.476 

0.930 

1.035 

0.828 

0.925    1.466 

other  words,  brasses  of  the  composition  here  used  dissolve  uni- 
formly as  such.  The  efficiency  of  corrosion  is  in  all  cases  about 
the  same  as  that  of  copper. 

Single  Potential  of  Brass. 

In  all  the  tables  in  which  the  single  potentials  of  copper  and 
zinc  are  given  it  is  observed  that  the  potentials  of  zinc  are  approxi- 
mately 0.1  volt  or  more  greater  than  those  of  copper.  -According 


36  KARL  GROVKR  STURDKVANT. 

to  the  theory  of  electro-deposition  it  is  impossible  for  two  metals 
to  be  deposited  simultaneously  when  their  potentials  differ  by  this 
amount.  Some  other  highly  influential  factor,  which  has  not  yet 
been  considered,  must  consequently  be  active  here. 

It  is  a  well-known  fact  that  strongly  electro-negative  metals 
can,  under  certain  conditions,  be  deposited  from  solutions  at 
potentials  much  lower  than  their  equilibrium  potentials.  For  ex- 
ample, sodium  may  be  deposited  from  an  aqueous  solution  of  its 
salts,  provided  mercury  be  used  as  cathode.  This  is  explained 


Lflf. 

^—  *—  j 

fCu 

_*  —  „ 

_—  -  —  -"• 

Curve  4 
7fl 

7,T 

I  7\. 

^~~^~       ' 

4  —  a. 

-^ 



—  - 

-I 

jO 

/«/,     ^^r 

hU 

0. 

*  
2           0 

4          0. 

>*.  

6           0. 

1 

8           Eq 

U  IV.  /Kfa  C 

OT.IL. 

PLATE  VII.     Influence  of  Sodium  Carbonate. 

1.     Potential  of  copper  anode  without  current. 
2.     Potential  of  zinc  anode  without  current. 
3.     Potential  of  cathode  with  current. 
4.     Percentage  of  copper  in  the  deposit. 

by  assuming  that  sodium  alloys  with  the  mercury,  and  the  result- 
ing alloy  shows  a  low  electrolytic  solution  pressure  for  sodium. 
It  appears  that  some  similar  action  must  take  place  in  the  deposi- 
tion of  brass  from  cyanide  solutions. 

It  is  known  that  zinc  and  copper  do  form  alloys.  The  poten- 
tials which  such  alloys  show  depend  upon  their  nature  and  com- 
position. A.  J.  Allmand  (Principles  of  Applied  Electrochemistry, 
p.  136)  pointed  out  that  the  potentials  of  those  alloys  which  are 
simply  solid  solutions  of  one  metal  in  another  vary  continuously 
with  composition  from  the  potential  of  one  metal  to  that  of  the 


THE;  ELECTRO-DEPOSITION   OF   BRASS.  37 

other.  In  case  the  two  metals  form  a  compound,  it  shows  a  char- 
acteristic potential  which  may  lie  between  the  potentials  of  the 
two  metals  or  may  even  exceed  the  potentials  of  either.  If  the 
alloy  consists  of  a  mixture  of  two  or  more  constituents  the  poten- 
tial which  the  alloy  shows  is  that  of  the  more  electro-negative 
constituent. 

TABLE  XVIII. 

Percentage  of  Copper  Efficiency  of  Corrosion 

62.3  104.8 

66.2  105.2 
70.7  103.8 
76.0  103.4 

81.4  103.2 
85.0  104.0 

It  seems  entirely  possible  that  a  thin  film  of  copper  is  first  de- 
posited, and  this  greatly  decreases  the  potential  necessary  for  the 
deposition  of  zinc.  This  depolarization  may  be  so  great  that  the 
simultaneous  deposition  of  the  two  metals  takes  place  from  solu- 
tions in  which  their  single  potentials  are  not  the  same.  The  two 
metals  thus  deposited  form  an  alloy,  the  potential  and  other  prop- 
erties of  which  depend  upon  its  nature  and  composition. 

In  order  to  obtain  some  information  on  the  potentials  of  brasses 
formed  in  this  way,  ten  samples  of  brass,  the  composition  of 
which  ranged  from  37.6  to  82.0  percent  of  copper,  were  prepared 
by  varying  the  acidity  of  the  stock  solution.  The  potentials  of 
these  brasses  were  then  measured  in  the  original  solution  and  the 
data  recorded  in  Table  XIX. 

TABLE  XIX. 

Percent  of  Copper  Potential 

37.6  1.005 

56.7  0.925 

62.3  0.928 

66.0  0.917 

69.5  0.978 
72.2  0.979 

79.1  0.984 
82.0  0.990 

Pure  Copper  0.914 

Pure  Zinc  1.080 

It  is  seen  from  this  table  that  the  potential  decreases  to  a  mini- 
mum with  increase  in  copper  to  66.0  percent  and  then  increases. 
Since  a  brass  deposit  which  contains  66.0  percent  of  copper  is  the 


38  EARI,  GROVER  STURDEVANT. 

most  satisfactory,  there  may  be  some  relation  between  this  low 
potential  and  the  nature  of  the  deposit.  The  potentials  of  these 
brasses  lie  between  the  single  potentials  of  copper  and  zinc  in  the 
same  solution  and  most  of  them  are  much  nearer  the  potential  of 
copper  than  the  potential  of  zinc.  These  results  lead  one  to 
believe  that  the  alloys  formed  by  electro-deposition  are  solid  solu- 
tions of  a  compound  of  copper  and  zinc  in  copper.  Further  ex- 
periments of  this  kind  would  doubtless  afford  valuable  informa- 
tion regarding  the  exact  nature  of  copper-zinc  alloys. 

Summary. 
As  a  result  of  this  work  the  following  statements  may  be  made : 

(1).  Increase  in  the  ratio  of  copper  to  zinc  in  the  solution 
increases  the  percentage  of  copper  in  the  deposit.  A  solution  in 
which  the  ratio  of  copper  to  zinc  is  4.2  gives  a  deposit  of  about 
65  percent  copper  (ratio  1.9). 

(2)  Solutions  of   high  metal  content  are   more   satisfactory 
than  dilute  solutions.    A  solution  containing  thirty-five  grams  of 
metal  per  liter,  in  the  above  ratio,  gives  satisfactory  deposits. 

(3)  Increase   in   temperature  decreases   cathode  polarization 
and  consequently  increases  the  percentage  of  copper  in  the  deposit. 

(4)  Increase  in  current  density  produces  a  gradual  decrease 
in  the  percentage  of  copper  in  the  deposit.     At  current  densities 
greater  than  0.3  ampere  per  sq.  dm.,  the  deposit  becomes  granular, 
non-adherent,  and  dull  in  color. 

(5)  Increase    in     free    cyanide    does    not    increase    anode 
efficiency,  but  does  decrease  cathode  efficiency.     Its  influence  on 
the  percentage  of  copper  in  the  deposit  is  variable. 

(6)  Slightly  acid  substances  increase  the  percentage  of  copper 
in  the  deposit.    A  weak  acid  may  be  used  in  place  of  any  of  the 
acid  substances  that  have  been  recommended. 

(7)  Slightly  alkaline  substances  decrease  the  percentage  of 
copper  in  the  deposit.    The  presence  of  slightly  alkaline  substances 
is  beneficial  in  that  it  improves  the  appearance  of  the  deposit. 

(8)  Neutral  substances  have  no  influence  on  the  deportment 
of  the  cyanide  brass  plating  solution. 


THE   ELECTRO-DEPOSITION   OF   BRASS.  39 

(9)  Brasses  which  vary  in  composition  from  62.3  to  85.0  per- 
cent of  copper  dissolve  as  such  anodically.    The  efficiency  of  cor- 
rosion is  about  the  same  as  that  of  copper. 

(10)  Decided  depolarization  of  zinc  by  copper  takes  place  and 
makes  possible  the  deposition  of  brass  from  solutions  in  which 
the  potentials  of  the  two  metals  are  not  equal. 

(11)  Electro-deposited    brasses    which    vary    in    composition 
from  37.6  to  82.0  percent  copper  give  nearly  the  same  potentials 
in  a  plating  solution.     These  potentials  are   nearer  to   that   of 
copper  than  to  that  of  zinc. 


UNIVERSITY  OF  CALIFORNIA  LIBRARY 
BERKELEY 

Return  to  desk  from  which  borrowed. 
This  book  is  DUE  on  the  last  date  stamped  below. 


MAY 


REC'D  LD 

NOV  24 


REC'D  LD 

MW  25'6A -1« 

LD  21-100m-9,'47(A5702sl6)476 


4587^4 


UNIVERSITY  OF  CALIFORNIA  LIBRARY 


' 


